Transposable elements (TEs) are selfish genetic entities that can rapidly sweep through populations, reaching high copy-number in eukaryotic genomes. More than 50 human genetic diseases are associated with TE insertions, or TE-mediated chromosomal rearrangements that occur recurrently in human populations. Misregulation of TEs, furthermore, is associated with tumorigenesis and tumor progression, suggesting a critical role for TE suppression in cancer resistance. The deleterious mutations that arise from TE activity exert strong selection for suppression by the host. A recent explosion of research on the Piwi- interacting RNA (piRNA) pathway has made the exciting discovery that these proteins act in concert with TE-derived small RNAs (piRNAs) to silence TEs in male and female germlines. The piRNA pathway has a conserved function across metazoa, yet surprisingly many piRNA proteins are the direct targets of positive selection. Using the model system Drosophila, my proposed research will elucidate both the protein and RNA contributions to evolutionary changes in piRNA mediated TE suppression. Specifically, I will: 1) Examine piRNA pathway function in D. melanogaster and D. simulans interspecific hybrids. Aberrant phenotypes observed in interspecific hybrids frequently indicate rapidly evolving components of the underlying biological process or pathway. By comparing piRNA protein localization, piRNA pools, and germline TE expression between interspecific hybrids and pure species, I will reveal aspects of the piRNA pathway that have diverged functionally. 2) Examine adaptive divergence of Aubergine, a critical piRNA pathway protein, between D. melanogaster and D. simulans. I will employ heterologous transgenics to distinguish specific functions of the piRNA protein Aubergine that have been targets of positive selection. This directed analysis of aubergine will help elucidate the selective force(s) that underlie adaptive protein evolution throughout the piRNA pathway. The proposed research will enhance our understanding of how selfish transposable elements are controlled by their hosts. Because activity of these elements contribute directly to the origin, onset, or progression of several human genetic diseases, this knowledge is critical to generating more effective treatment and prevention